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Resin Casting

2K views 9 replies 5 participants last post by  ckucia 
#1 ·
I have some casting materials on order that I intend to experiment with for reproducing a tail light array.

Wondering if anyone here has experience with resin casting and could offer any hints. I've done some research on the web. Here's what I've come up with...

I'm planning on using a silicon RTV rubber for the mold. I will probably brush it on in couple of layers, then strengthen it from the back by covering with a base of plaster. I plan on doing the two halves in steps - coat the outside with RTV, leave the part in the RTV and set the plaster backing, coat the exposed edges of the RTV with release agent and coat the inside with RTV, coat the existing plaster with releas agent and set the plaster backing on the other side. That should give me a decent two-piece mold that is very rigid.

I'm wondering if plaster is the best material. Its cheap and easy to work with, and its also very rigid. Its also fairly heavy, which will help to "pressurize" the casting. However, it can be dusty, and I'm worried about potential shrinkage. I guess pouring it in thin layers might be better, although I've considered that perhaps I should first coat the RTV with epoxy, then reinforce the epoxy with aluminum or other metal pieces to make it rigid.

I'm planning on using a UV-resistant clear polyurethane resin for the actual lens.

The lens is pretty large and curves inward at both ends. To complicate things, its composed of two colors of plastic that were bonded by the OEM. Since I can't risk destroying the lens by trying to take the two colors apart, I plan on making a casting out of PU, then cutting it where appropriate to isolate what will be the one color (red). I can then use this to make a new mold for just that color. When I have that piece cast, I can them put it into the full mold and pour the other color (clear) resin around it which should (if I'm right) give me one large piece.

The lens itself is pretty thin, so I'm hoping I can get away without using vacuum or pressure, although I will probably set up a small vessel to pressurize or vacuum the resin before pouring.

I know that the UV-resistant PU will not offer me the same long-term performance as acrylic would, but acrylic, from what I've read, requires another level of complication to make decent parts. Since these tail lights aren't available from the OEM, and once I have the molds I can turn out a couple of sets, I'm willing to trade some longevity for ease of production.

Any hints/tips/suggestions?
 
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#2 ·
You'll have a hard time getting anything to stick to the rtv. The look and durability of your product can be increases by spraying it with a urethane clearcoat when finished. This is interesting, could you post some pics of the procedure. You may want to consider using polyester body filler instead of the plaster, the setup time would be faster and it could be painted if dust is a concern. Bob
 
#3 ·
badbob,

I had thought of clearing the resulting parts. Might give an extra measure of scratch protection and I can be confident that finish was designed to withstand typical auto exterior exposure. Do you know if clear has any UV protection? If it did, then that would be even better.

Polyester body filler is a great idea - I've already got a big container of it on the shelf. It sets up quick too!

I'll try and get some pics taken. I still have to take my old lenses apart, and that's going to be a very time-consuming and harrowing experience. I'm trying to do as much legwork as I can before taking them apart (although I've looked inside using a small mirror through the bulb holes). Once I start that, I'm pretty committed, especially if I damage something. I've got some other, simpler lenses lying around for some other car. I'm going to use those as a test before I take apart the ones I intend to reproduce.

These lenses go for anywhere from $300-$500 (each side) on ebay, and that's for used ones.
 
#4 ·
All premium urethane automotive clears have UV protection built into them, the ones with a high poly content are the toughest. I've sprayed quite a few older plastic lenses in an effort to restore them and it seems to work really well-gives them an as new appearance and the adhesion is excellent. Although the oem stuff is I'm sure made of a different material I'm sure spraying clear on your product would work just as well. What model car are these going to be for?
 
#5 ·
I should have mentioned that...they're for a Cadillac Allante.

I believe the OEMs generally use acrylic for their lenses. However, the Allante is an oddball in a lot of ways and I wouldn't be surprised if something else was used. The taillight arrays were originally produced by a company in Italy, that was presumably sourced by Pininfarina - the company that built the bodies. A couple of people have tried contacting that company to see if they'd consider a run, or at least determine if the molds still exist, but without success.

I'm pretty confident I can produce "something". Whether or not it looks good enough is a different story. I have three major concerns: bubbles and voids in the resin, resin shrinkage (another reason not to use acrylic), and maintaining the mold half tolerances to insure I get good clarity (no warping/waving) on the lens. With the last, I could probably polish out any slight variations, but if too radical, I won't be able to get around it.


I'm also wondering if the silicone is the best mold-making material. Experimentation will tell. It appears to have excellent detail-rendering, but being slightly flexible, I might not be able to maintain the detail on a large part. Luckily, being thin, I shouldn't have too much of a problem with the resin shrinking. A lot will depend on the viscosity of the resin. I might also try an epoxy mold which might be better if I have to put the whole mold in a vacuum or pressure chamber to ensure a good casting.

I've also considered thermoforming the clear portion out of polycarbonate, or even acrylic, and mating it to a cast red portion (the red part has the "pyramids" that give it a reflective quality on the back). I'm not sure that would be worth the tremendous effort to get an exact match to the red part, and then I have to consider how to bond them, that's why I thought casting the clear around the red in the same material would be the best bet, even if it requires some degree of polishing to get the clarity. I haven't ruled out thermoforming the clear and casting the red around it, but I'm hoping that I can do the whole thing with casting.

I've been thinking about this for a long time, but I'm still a novice at it. Its amazing what's available though. It might even turn into a small side-business someday. If I can make something this complicated, then most other car lenses or other reproduction parts would be simple.
 
#6 ·
i have a little expierence with casting resins and rubber molds but i wouldn't call myself an expert. i have tried epoxy, polyurethane and polyester casting resins and found epoxy to be the best atleast for what i need. the polyurethane parts that i cast seemed fime at room temp but when they got warm, say if they say in the sun awhile the part got very soft so the poly wasnt very stable. polyesters are very hard to controll the cure speed. i think the epoxy works awesome. very user friendly. unless you find a resin thats water thin you are going to have a problem with bubbles. i have a small vacuum chamber setup here to draw out any airbubbles before pouring. it would be impossible without it. both the epoxy and poly are thicker and need to be degassed. the polyesters are thinner and may work but i doubt it. what you are trying to do isn't really a simple thing as i'm sure you are finding out. people go to college for years to learn this stuff. i just figured out what i needed to know by trial and error but there is a big learnig curve. its definately an artform. silicone seems to be the way to go in the mold rubber world. best thing i can tell you is goto www.smooth-on.com they have just about everything you need except clear epoxy which you can get from www.polytek.com. not sure if the epoxy is uv stable but like bob said you can clearcoat it. another thing you can do instead of splitting the two colors is cast the whole thing clear then spray candy apple red over the red area. you'll never know its not red all the way through. i have candied many sets of taillights and turnsignals different colors and even black candy to smoke them. works great
 
#7 · (Edited)
Plastics forming

This might help..I worked with plastics at one time and I think if I actually saw one of what you are trying to make I might hae some input..The following is what we used to heat form pieces of clear for canopies and various parts..If the curvature is not too great Nothing more pronouched than say the curve of a softball)this stuff could work and holds up well in service..

Availability: Available in colorless sheet in thickness of 1/16" • 3/32" • 1/18" • 3/16" • 1/4"
Sheet size: 4' x 8'. We cut to size.


Impact Strength: Has many times the impact strength of glass and standard acrylic sheet.


Light Weight: Weights half as much as glass.


Surface Hardness: Good surface hardness helps reduce damage during fabrication and extends service life.


Strength & Stresses:Tensile strength is 8,900 psi at room temperature.


Heat Resistance: Keeps its rigid shape up to 160 degrees F.


Light Transmission: Excellent light transmission. Material resists yellowing and fading after prolonged sun exposure. Light transmittance is greater than 91% (1/8" thickness). It retains high light transmitting properties for many years.


Formability: Forming temperature range is 270 degrees F to 350 degrees F.


Chemical Resistance: Inorganic alkalies, diluted acids, aliphatic hydrocarbons, aromatic solvents, alcohols, chlorinated hydrocarbons, and lacquer thinners.
Click here for more information about ACRYLITE PLUS®

One would have to make a forming die and pull the parts from that..Since cad allenate is in short supply around here I have not a clue as to what the shape is you are after..I have made some parts by heating and pulling the plastic over a die..Seems to me to be a whole lot easier than making and pouring a resin type lense..

worked out fine..and like the guy said just paint the inside with some candy paint..

Just a suggestion..go to tapplastics.com for more info..


OMT
 
#8 ·
If this lens you want to reproduce is bigger than about 3" by 3" then I think you are in for a couple of problems.

I am a mold maker by trade and back in the day when we use to use duplicators to cut plastic injection molds we made a lot of models.

The ones that we made from existing parts we always used an epoxy resign that we called black magic.

The first step was to set up the part so that you could build a box around it that would contain the resign and the part while being poured.

Next step place the part in the box and level it up so when you pour the epoxy in it will flow out and cover the part with the least amount of resign required to make sure the thinest part of the resign will ent up about 3/4 of an inch thick.

Next step is to take plasticine or modeling clay and put it all around the outside of the part so that it makes a parting line where everything that is sticking above the modeling clay will pull out of the half of the mold you are making.

Next step spray plasticine inside of box and part you are casting with release agent.

Ok now you can pour in the black magic. Don't pour it on top of the part though pour it onto the lowest part of the parting line that you created and keep pouring in this location until the part is covered by 3/4 of an inch. ( pouring in this method allows the epoxy to flow out and eliminates air bubles in the epoxy that could leave a void in the molding surface.

Wow one half done.

Once this haf cures take it out of the box and remove the modeling clay from the parting line. Do not remove the part.

Build a box around the half of the mold you now have spray inside of box part and parting line with release agent and fill this side of the mold with epoxy. again a minimum of 3/4 thick at the thinest point.

When it hardens drill and ream a couple of holes thru the two halves to use as alignment holes.

Split the two halves apart

install dowels in the holes and you have a mould.

The problem with the mold is that it will be about .006" per inch smaller than the original part.

so a part that is 10 inches long will end up about 1/16 of an inch smaller than the original.

If the resign that you pour into the mold shrinks also then you will be smaller again depending on the shrink ratio.

if it was .006 per inch you would end up with a 10 inch part that actualy measures about 1/8 of an inch smaller.

ATB
Ric
 
#9 ·
this has nothing to do with the question being asked but has to do with casting and i thought is was really cool. was watching discover a while back and they had a special on something about toyota. anyway they showed you this machine they have. its a big tub of clear resin, about 500 gallons. the resin gets hard and cures with light, UV etc. over top the tub is a cnc lazer. they just draw out a part on the cad program, this lazer shoots a beam into this tub curing the resin in a pinpoint spot down deep in the tub. the laser keeps moving around curing the resin. 1/2 hour later you just pull your plastic part out of this vat of resin. thought that was a crazy idea.
 
#10 ·
Many thanks for the replies.

I had seen clear epoxy available, that might be something to investigate further. I believe it was not UV safe when I researched it, but at the time I didn't realize clearing the part would give that much protection.

The candy paint is an interesting idea. That could make the whole project a lot easier.

I wish I could just form the entire part over a buck but unfortunately, the "red section" has that texture on the back that makes it reflective - sort of a "sea" of pyramids. To comlicate things, the strip is in roughly 4" long sections where the pyramids align in a slightly different direction so they reflect from a different angle. So, I need to be able to control both the smooth outside and the pyramids inside.
 
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